1. Field of the Invention
The present invention relates to laser systems and in particular to minimizing damage to optical materials, such as crystals, that are used in laser systems.
2. Related Art
Optical materials are key elements of modern-day laser systems. Common applications for optical materials include fabrication of basic optical components (e.g. windows, mirrors, polarizers, etc.) as well as nonlinear optical devices for frequency mixing (e.g. harmonic generation, parametric generation/amplification), Raman amplification, Kerr-lens modelocking, electro-optic modulation, and acousto-optic modulation.
When exposed to laser radiation, physical properties of these optical materials may change with time as a result of interaction between light and matter. Such changes may adversely affect optical properties including, but not limited to, transmittance, reflectivity, and refraction indices. These adverse changes are generally referred to as material damage. The material lifetime, i.e. the amount of time in which the material is serviceable for its intended purpose within a range of accuracy, is typically defined based on a maximum acceptable change in one or more of these parameters.
Laser-induced damage of optical materials is one of the main factors affecting material lifetime, thereby limiting performance of laser systems. Laser-induced damage has been subject to extensive research, which has focused on (i) improving the quality of the existing optical materials and development of new compounds that possess high laser damage tolerance and/or (ii) optimization of operating conditions to mitigate laser-induced damage. This optimization of operating conditions has become increasingly important as the growing demand for high power DUV (deep ultraviolet light having a wavelength λ<300 nm) lasers imposes such extreme material requirements that conventional damage tolerance improvement approaches quickly become insufficient, too costly, and/or impractical.
The most common technique used to mitigate laser-induced damage is thermal annealing. It is widely employed as a method of reducing laser-induced damage because it works for many different types of damage in various materials and is relatively easy to implement. Unfortunately, efficient thermal annealing can require a high temperature, which when used during real-time laser system operation can pose significant disadvantages.
First, presence of a high temperature inside a laser system is highly undesirable because thermal gradients can cause air fluctuations that result in output beam instability and increased outgassing intensifies photocontamination. Second, at short (e.g. DUV) wavelengths, when photon energies approach band gaps of common optical materials, phonon-assisted absorption becomes significant and increases exponentially with temperature. This absorption can lead to decline in overall system performance, for example, one can observe decreased conversion efficiency when applied to nonlinear frequency conversion. Moreover, this phonon-assisted absorption may exacerbate other laser-induced damage to the optical material, thereby potentially negating the positive effects of annealing.
Note that in certain situations, the temperature of the optical material needs to be precisely controlled for reasons unrelated to material lifetime. For example, in non-critical phase matching (for certain wavelengths combinations only), the phase mismatch can be minimized by adjusting the crystal temperature such that the phase velocities of the interacting beams are equal. Therefore, for such situations, the application of thermal annealing is limited, if at all possible.
Therefore, a need arises for a technique of minimizing laser-induced damage in optical materials that overcomes the drawbacks of conventional annealing.